This invention relates to computed tomography (CT) systems and specifically to a helical scanning CT system in which the imaged object is concurrently translated during the acquisition of tomographic projections.
In a computed tomography system, an x-ray source is collimated to form a fan beam with a defined fan beam angle. The fan beam is typically oriented to lie within the x-y plane of a Cartesian coordinate system, termed the "gantry plane", and is transmitted through an imaged object to an x-ray detector array oriented within the gantry plane. The detector array is comprised of an array of detector elements each of which measures the intensity of transmitted radiation along a ray projected from the x-ray source to the particular detector element. The intensity of the transmitted radiation is dependent on the attenuation of the x-ray beam along that ray by the imaged object.
The center of the fan beam and its direction of the fan beam is identified by a fan beam axis.
The x-ray source and detector array may be rotated on a gantry within the gantry plane and around a center of rotation within the imaged object so that the angle at which the fan beam axis intersects the imaged object may be changed. At each gantry angle, a projection is acquired comprised of the intensity signals from each detector element. The gantry is then rotated to a new angle and the process is repeated to collect a number of projections along a number of gantry angles to form a tomographic projection set.
The acquired tomographic projection sets are typically stored in numerical form for later computer processing to "reconstruct" a slice image according to reconstruction algorithms known in the art. A projection set of fan beam projections may be reconstructed directly into an image by means of fan beam reconstruction techniques, or the intensity data of the projections may be sorted into parallel beams and reconstructed according to parallel beam reconstruction techniques. The reconstructed tomographic images may be displayed on a conventional CRT tube or may be converted to a film record by means of a computer controlled camera.
A typical computed tomographic study involves the acquisition of a series of "slices" of an imaged object, each slice parallel to the gantry plane and having a slice thickness dictated by the width of the detector array, the size of the focal spot, the collimation and the geometry of the system. Each successive slice is displaced incrementally along a z-axis, perpendicular to the x and y axes, so as to provide a third spatial dimension of information. A radiologist may visualize this third dimension by viewing the slice images in order of position along the z-axis, or the numerical data comprising the set of reconstructed slices may be compiled by computer programs to produce shaded, perspective representations of the imaged object in three dimensions.
As the resolving power of computed tomography methods increases, additional slices are required in the z-dimension. The time and expense of a tomographic study increases with the number of slices required. Also, the longer scan times necessary to acquire more slices increases the discomfort to the patient who must remain nearly motionless to preserve the fidelity of the tomographic reconstructions. Accordingly, there is considerable interest in reducing the time required to obtain a slice series.
The time required to collect the data for a series of slices depends in part on four components: a) the time required to accelerate the gantry to scanning speed, b) the time required to obtain a complete tomographic projection set, c) the time required to decelerate the gantry, and d) the time required to reposition the patient in the z-axis for the next slice. Reducing the time required to obtain a full slice series may be accomplished by reducing the time required to complete any of these four steps.
The time required for acceleration and deceleration of the gantry (a and c) may be avoided in tomographic systems that use slip rings rather than cables to communicate with the gantry. The slip rings permit continuous rotation of the gantry and avoid the need for acceleration and deceleration. Hereafter, it will be assumed that the CT systems discussed are equipped with slip rings or the equivalent to permit continuous rotation.
The time required to acquire the tomographic data set (b) is more difficult to reduce. Present CT scanners require on the order of one to two seconds to acquire the projection set for one slice. This scan time may be reduced by rotating the gantry at a faster speed. However, a higher gantry speed, in general, will decrease the signal-to-noise ratio of the acquired data by the square root of the factor of rotational rate increase. This may be overcome to some extent by increasing the radiation output of the x-ray tube, but is subject to the power limits of such devices.
Finally, a reduction in patient repositioning time (d) may be accomplished by translating the patient in the z-axis concurrently with the rotation of the gantry. The combination of continuous patient translation along the z-axis during the rotation of the gantry and acquisition of projection data has been termed "helical scanning" and refers to the apparent path of a point on the gantry with respect to a reference point on the imaged body. As used herein, "helical scanning" shall refer generally to the use of continuous translation of the patient or imaged object during the acquisition of tomographic imaging data, and "constant z-axis scanning" shall refer to the acquisition of the tomographic data set without translation of the patient or imaged object during the acquisition period.
Continuous translation of the imaged object during scanning shortens the total scanning time required for the acquisition of a given number of slices by eliminating the length of time normally required for repositioning the patient between scans. However, helical scanning introduces certain errors in the acquired tomographic projection sets. The mathematics of tomographic reconstruction assumes that the tomographic projection set is acquired along a constant z-axis slice plane. The helical scan path clearly deviates from this condition and this deviation results in image artifacts in the reconstructed slice image if there is any significant change in the object in the z-axis. The severity of the image artifacts depends generally on the "helix offset" in the projection data, measured as the z-axis difference between the scanned volume elements of the imaged object and the z axis value of the desired slice plane. Errors resulting from helical scanning will be referred to collectively as "skew" errors.
Several methods have been used to reduce skew errors in helical scanning A first approach disclosed in U.S. Pat. No. 4,630,202 issued Dec. 16, 1986, reduces the pitch of the helical scan and then averages the projection data of consecutive 360.degree. tomographic projection sets. The effect is equivalent to using a detector array with a larger width along the z axis, which also moves less in the z direction during a rotation of the gantry, i.e. with a lesser scanning pitch. Skew errors are reduced using this method, but at the expense of additional scanning time necessitated by the lower scanning pitch. Thus, this method reduces, to some extent, the advantages to be gained by helical scanning.
Skew errors at the ends of the tomographic projection set may be reduced in conjunction with this approach by changing the weighting of the last and first projections of the consecutive 360.degree. tomographic projection sets in the "averaging" process to give greater weight to the projection closest to the slice plane.
A second approach disclosed in U.S. Pat. No. 4,789,929 issued Dec. 6, 1988, also applies weighting to the projections of combined, consecutive 360.degree. tomographic projection sets, but the weighting is a function of the helix offset of each projection at the given gantry angle. This approach of interpolating over 720.degree. generally increases partial volume artifacts. Partial volume artifacts are image artifacts arising when certain volume elements of the imaged object contribute to only some of the projections of the projection set.
A third approach, described in co-pending U.S. patent application Ser. No. 07/435,980, entitled: "Extrapolative Reconstruction Method for Helical Scanning" and assigned to the same assignee as the present invention, uses a half-scanning technique to reduce the table motion during the acquisition of each slice. Projection data is acquired over 360.degree. of gantry rotation and interpolated to a slice plane. The reduced gantry motion corresponds to reduced table motion and hence certain helical scanning artifacts are reduced.
In U.S. Pat. No. 5,090,037 entitled: "Helical Scanning Computed Tomography with Tracking X-ray Source", assigned to the same assignee as the present invention, skew error is reduced by translating the x-ray beam with translation of the imaged object. This translation may be accomplished by, for example, a movable collimator which sweeps the angle of the fan beam to track a particular volume element of the imaged object. Although this technique is successful in reducing the effects of helix offset, it has previously been assumed that it cannot be used for sequences of thick slices where there is the possibility that the edges of the fan beam will move beyond the edge of the detector.